Aluminum hydroxide coagulant recovery from water/wastewater treatment sludge

ABSTRACT

A method for recovery of aluminum hydroxide Al(OH) 3  from an aluminum enriched water/wastewater treatment sludge is disclosed. The method includes the steps of: adding a hydrated lime slurry to the aluminum enriched water/wastewater treatment sludge to form an alkaline sludge; adding sodium carbonate Na 2 CO 3  to the alkaline sludge to form a Na 2 CO 3  treated sludge; forming a first supernatant from the Na 2 CO 3  treated sludge of step b) containing NaAl(OH) 4 ; introducing CO 2  to the first supernatant to form a precipitate of Al(OH) 3  and a second supernatant containing NaHCO 3 ; and recycling at least a portion of the NaHCO 3  from the second supernatant back to the alkaline sludge of step a).

FIELD OF THE INVENTION

The present invention relates to methods and processes for aluminumhydroxide coagulant recovery from aluminum enriched water/wastewatertreatment sludge under alkaline conditions, in particular, forrecovering aluminum hydroxide from the sludge using hydrated limeCa(OH)₂, sodium carbonate Na₂CO₃ and CO₂ gas.

BACKGROUND

Aluminum hydroxide (alum) is the most common coagulant used in water andwastewater treatment. The main purpose of alum in these applications isto improve the settling of suspended solids and color removal. Alum isalso used to remove phosphate from wastewater treatment effluent. In allthese applications, aluminum enriched sludge is generated. Alum in thesludge generated during these processes causes difficulties with sludgedewatering and disposal because alum traps water. Therefore, aluminumrecovery from water and wastewater treatment sludge has severalsignificant advantages: (1) improving sludge dewaterability, (2)reducing sludge mass or weight by dewatering the sludge, and (3) cuttingdown the cost of alum by reusing the recovered alum.

There are limited options for recovery of alum from sludge containinghigh concentration of CaCO₃, CaSO₄, and Mg(OH)₂. The most widelyaccepted method for aluminum recovery is acidulation of the sludge.Exemplary examples of aluminum recovery under acidic circumstanceinclude CN105948442, TW201226577, KR20040088093, U.S. Pat. Nos.5,304,309, 6,495,047, US2013319941, JP2002113472, RU2133225, U.S. Pat.Nos. 4,130,627, 3,959,133, etc.

Attempts have been made to recover aluminum hydroxide under alkalineconditions. For example, EP2017225 to Olsson et al. discloses a processfor production of an aluminum hydroxide containing filter cake fromaluminum containing waste waters, in which acidic and alkaline wastewaters containing aluminum are mixed in a ratio resulting in a pH of9-11 to obtain mixed waste waters. The mixed waste waters are allowed toreact with each other during a retention time of at least 5 hoursresulting in an aluminum hydroxide containing slurry, which is dewateredto obtain an aluminum hydroxide containing filter cake.

EP2452924 to Rossi et al. discloses a method for the treatment of wasteliquids originating from plants for the treatment of aluminum thatincludes mixing a first alkaline eluate (B) that contains sodiumhydroxide (NaOH) and sodium aluminate (NaAlO₂), and a second acid eluate(A) which contains sulfuric acid (H₂SO₄) and aluminum sulfate(Al₂(SO₄)₃) to achieve the forming of an aluminum hydroxide precipitate(Al(OH)₃) on the bottom of a reactor upon reaching a pH value of thecontent of the reactor between 7.5 and 9.0.

Masschelein et al. (W. J. Masschelein, R. Devleminck and J. Genot, “TheFeasibility of Coagulant Recycling by Alkaline Reaction of AluminumHydroxide Sludges”, Water Research, vol. 19, No. 11, 1985, p 1363-1368)disclose recovery of aluminum from a waterworks sludge by alkalizationwith NaOH or Ca(OH)₂ under laboratory conditions. Even though it waspossible to recover 80% of the aluminum the authors recommendedrecycling less than 50% of the total dosing rate because of impurityproblems. Lime was more effective than sodium hydroxide for removingheavy metals from the recovered coagulant.

In addition, cost is also an issue to be considered for aluminumhydroxide coagulant recovery industry.

Thus, a need remains for effective recovery of aluminum hydroxide fromwastewater treatment sludge.

SUMMARY

There is disclosed a method for recovery of aluminum hydroxide Al(OH)₃from an aluminum enriched water/wastewater treatment sludge, the methodcomprising the steps of:

-   -   a) adding a hydrated lime slurry to the aluminum enriched        water/wastewater treatment sludge to form an alkaline sludge;    -   b) adding sodium carbonate Na₂CO₃ to the alkaline sludge to form        a Na₂CO₃ treated sludge;    -   c) forming a supernatant from the Na₂CO₃ treated sludge of        step b) containing NaAl(OH)₄;    -   d) introducing CO₂ to the supernatant to form a precipitate of        Al(OH)₃ and a supernatant containing NaHCO₃; and    -   e) recycling at least a portion of the NaHCO₃ from the        supernatant back to the alkaline sludge of step a).

In some embodiments the pH of the aluminum enriched water/wastewatertreatment sludge is approximately 7.

In some embodiments the pH of the alkaline sludge ranges fromapproximately 11.5 to 12.

In some embodiments the pH of the supernatant ranges from approximately6.5 to 7.5.

In some embodiments the pH of the supernatant is approximately 7.

In some embodiments the method further comprises a step of retaining thealuminum enriched water/wastewater treatment sludge for a contact timewith the hydrated lime slurry sufficient in length to inactivate atleast a portion of bacteria and viruses present in the sludge during thestep a) of forming the alkaline sludge.

In some embodiments the contact time is about 2 to 5 hours.

In some embodiments the aluminum enriched water/wastewater treatmentsludge contains up to 5% solids.

In some embodiments the solids include aluminum hydroxide.

In some embodiments the method further comprises a step of thickeningthe aluminum enriched water/wastewater treatment sludge to increase asolid content of the sludge.

In some embodiments the thickened aluminum enriched water/wastewatertreatment sludge contains up to 5% solid content.

In some embodiments the solid content in the thickened aluminum enrichedwater/wastewater treatment sludge includes aluminum hydroxide.

In some embodiments the pH of the thickened aluminum enrichedwater/wastewater treatment sludge is approximately 7.

In some embodiments the method further comprises the steps of:

-   -   recycling a slurry phase of the alkaline sludge;    -   dewatering a waste sludge from the slurry phase to form a        dilution water with a high pH value, preferably pH 11.5 or        greater; and    -   mixing the dilution water with a hydrated lime to form the        hydrated lime slurry.

In some embodiments the method further comprises a step of addingMg(OH)₂ to the alkaline sludge to purify the alkaline sludge.

In some embodiments the method further comprises a step of addingMg(OH)₂ to the alkaline sludge in an amount sufficient to coagulatenatural organic matter present in the alkaline sludge.

In some embodiments the hydrated lime slurry contains 5-10% Ca(OH)₂.

In some embodiments the alkaline sludge contains calcium alum inateCaAl₂(OH)₈ according the reaction: 2Al(OH)₃+Ca(OH)₂→CaAl₂(OH)₈.

In some embodiments NaAl(OH)₄ in the supernatant is formed by thefollowing reactions:

CaAl₂(OH)₈+Na₂CO₃→2NaAl(OH)₄+CaCO₃  a)

Ca(OH)₂+Na₂CO₃→2NaOH+CaCO₃  b)

2NaOH+CaAl₂(OH)₈→2NaAl(OH)₄+Ca(OH)₂.  c)

In some embodiments NaHCO₃ in the supernatant is formed by the followingreactions:

CO₂+H₂O→H₂CO₃*  a)

H₂CO₃→H⁺+HCO₃ ⁻  b)

HCO₃ ⁻→H⁺+CO₃ ²⁻  c)

NaAl(OH)₄+H₂CO₃*→NaHCO₃+Al(OH)₃(s)+H₂O.  d)

In some embodiments NaHCO₃ recycled back to the alkaline sludge formsNaAl(OH)₄ by the following reactions:

Ca(OH)₂+NaHCO₃→NaOH+CaCO₃+H₂O

2NaOH+CaAl₂(OH)₈→2NaAl(OH)₄+Ca(OH)₂.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 is a block flow diagram of an exemplary embodiment of a systemfor recovery of aluminum hydroxide coagulant from an aluminum enrichedwater/wastewater treatment sludge.

DESCRIPTION OF PREFERRED EMBODIMENTS

Disclosed are methods and processes for recovery of aluminum hydroxide(Al(OH)₃) coagulant from an aluminum enriched water/wastewater treatmentsludge under alkaline conditions. More specifically, aluminum hydroxide(or alum) is recovered with hydrated lime slurry (Ca(OH)₂), soda ash(Na₂CO₃) and carbon dioxide (CO₂) gas from the aluminum enrichedwater/wastewater treatment sludge. In the disclosed method, the alum isconverted to aluminate Al(OH)⁴⁻. Aluminum hydroxide is recovered fromthe alum containing sludge by alkali treatment using a combination oflime and sodium carbonate. Aluminum hydroxide is dissolved in the formof CaAl₂(OH)₈ and further NaAl(OH)₄. The solubility of other mineralsludge components is low at high pH values. In order to eliminate thecarryover of other dissolved species and organic matter, the dissolvedspecies and organic matter are removed by coagulation with Mg(OH)₂without lowering the pH. The dissolved aluminum hydroxide isre-precipitated by lowering the pH using carbon dioxide gas. The liquidphase that is saturated with sodium bicarbonate (NaHCO₃) is reused toreduce Na₂CO₃ feedstock use for alum recovery.

The disclosed methods for recovery of aluminum hydroxide Al(OH)₃coagulant from the aluminum enriched water/wastewater treatment sludgecomprise the steps of:

-   -   a) adding a hydrated lime slurry to the aluminum enriched        water/wastewater treatment sludge to form an alkaline sludge;    -   b) adding sodium carbonate Na₂CO₃ to the alkaline sludge to form        a Na₂CO₃ treated sludge;    -   c) forming a first supernatant from the Na₂CO₃ treated sludge of        step b) containing NaAl(OH)₄;    -   d) introducing CO₂ to the first supernatant to form a        precipitate of Al(OH)₃ and a second supernatant containing        NaHCO₃ (preferably saturated); and    -   e) recycling at least a portion of the NaHCO₃ from the second        supernatant back to the alkaline sludge of step a) to lower        Na₂CO₃ feedstock consumption by the method

FIG. 1 is a block flow diagram of an exemplary embodiment of a systemfor recovery of aluminum hydroxide coagulant from an aluminum enrichedwater/wastewater treatment sludge. As shown, the aluminum enrichedwater/wastewater treatment sludge, which usually contains less than 1%solids and has a pH of about 7, is fed to a sludge thickener 102 toincrease its solid content. Here, the solid content may have an aluminumcontent including aluminum hydroxide or alum. The sludge may bethickened to increase its solid content up to 5%. If the solid contentin the sludge is initially in a range of 4-5%, the thickening step maybe bypassed. The thickened sludge also generally has a pH ofapproximately 7. Excess liquid or water after thickening the sludge maybe sent to a sewage treatment plant for further purification. The sludgethickener 102 may be a mixing tank having an inlet for adding thealuminum enriched wastewater treatment sludge in, a first outlet forsending the excess liquid or water out to the sewage treatment plant anda second outlet for delivering the thickened sludge out to an aluminumrecovery reactor 104 for aluminum recovery.

The aluminum recovery reactor 104 may be a reactor tank or a clarifier.A hydrated lime slurry from a container 108 is pumped to the aluminumrecovery reactor 104 where the pH of the thickened sludge is raised,thereby forming an alkaline sludge. The solubility of aluminum hydroxideAl(OH)₃ increases with increasing pH. The hydrated lime slurry,preferably 5-10% calcium hydroxide Ca(OH)₂ slurry, is used to raise thepH of the thickened sludge to approximately 11.5-12. Under thiscircumstance, aluminum hydroxide Al(OH)₃ is dissolved in the form ofaluminate (Al(OH)⁴⁻) in the alkaline sludge. The solubility ofaluminate, however, is limited by the presence of calcium ions that formcalcium aluminate CaAl₂(OH)₈. Thus, the hydrated lime slurry, calciumhydroxide Ca(OH)₂, may react with some portion of aluminum hydroxideAl(OH)₃ in the thickened sludge forming calcium aluminate CaAl₂(OH)₈,according to the equation (I)

2Al(OH)₃+Ca(OH)₂→CaAl₂(OH)₈  (I)

Raising the pH of the thickened sludge may also effectively disinfectthe thickened sludge. Given a sufficient contact time between thethickened sludge and the hydrated lime slurry, preferably 2 to 5 hours,some or most bacteria and viruses in both liquid and solid phases of thethickened sludge may be completely inactivated. Thus, an additionalsludge disinfection treatment process may be unnecessary, which resultsin cost-saving.

Calcium aluminate CaAl₂(OH)₈ has relatively low solubility that limitsthe alum recovery efficiency. In order to lower calcium concentration, asufficient quantity of soda ash or sodium carbonate, Na₂CO₃, is added tothe aluminum recovery reactor 104 to precipitate calcium as athermodynamically stable product, calcium carbonate CaCO₃. Upon theaddition of the soda ash Na₂CO₃, calcium aluminate CaAl₂(OH)₈ isconverted to sodium aluminate NaAl(OH)₄ and calcium precipitates outfrom the alkaline sludge as calcium carbonate CaCO₃. The soda ash orsodium carbonate Na₂CO₃ may be added in excess and reacts withundissolved calcium hydroxide Ca(OH)₂ in the slurry phase to generatesodium hydroxide NaOH and calcium carbonate CaCO₃. The generated sodiumhydroxide NaOH increases calcium aluminate CaAl₂(OH)₈ reaction. Thus,upon the addition of the soda ash Na₂CO₃ in the aluminum recoveryreactor 104, most Al(OH)₃ is converted to sodium aluminate NaAl(OH)₄which has much higher solubility than calcium aluminate CaAl₂(OH)₈.Sodium aluminate NaAl(OH)₄ is formed in the aluminum recovery reactor104 through the following reactions given in Equations (II)-(IV):

CaAl₂(OH)₈+Na₂CO₃→2NaAl(OH)₄+CaCO₃(solid)  (II)

Ca(OH)₂+Na₂CO₃→2NaOH+CaCO₃(solid)  (III)

2NaOH+CaAl₂(OH)₈→2NaAl(OH)₄+Ca(OH)₂  (IV)

The alkaline sludge in the aluminum recovery reactor 104 now contains aliquid phase with sodium aluminate NaAl(OH)₄ and a solid phase or asettled sludge phase with a precipitated calcium content. The solidphase or the settled sludge of the alkaline sludge formed herein maystill contain some of alum. Thus, the settled sludge from the aluminumrecovery reactor 104 may be fed to a sludge storage tank 110 andrecycled back to the aluminum recovery reactor 104 to maximize aluminumrecovery.

During alum extraction in the alkaline sludge it is possible that otherconstituents of the sludge are also solubilized. However, most of theseelements have very low solubility in a pH ranging from 11.5 to 12compared to aluminum hydroxide Al(OH)₃. For aluminum recovery fromsludge having high portion of organic substances presented in thealkaline sludge, some of the organic substances may dissolve into theliquid phase of the sludge at high pH values (e.g., around 11.5 to 12herein). However, calcium carbonate CaCO₃ precipitate is known to inducesweep coagulation by entrapping dissolved and suspended organics.Therefore, the concentration of organic substances is generally low inthe alkaline sludge. In case the organic substances are not removed,Mg(OH)₂ may be added to the alkaline sludge for high pH coagulation toremove the organic substances in the liquid phase.

The liquid phase or the supernatant of the alkaline sludge from thealuminum recovery reactor 104 now predominantly consists of NaAlOH₄ andhas a high pH value of about 11.5 to 12. The supernatant from thealuminum recovery reactor 104 is forwarded to a CO₂ contact reactor 106where CO₂ gas is added to adjust the pH of the supernatant down to arange of 6.5-7.5, preferably approximately 7. The supernatant from thealuminum recovery reactor 104 may be pumped to the CO₂ contact reactor106, or may be fed into the CO₂ contact reactor 106 by gravity dependingon the design of the aluminum recovery reactor 104 and the CO₂ contactreactor 106. The CO₂ in the CO₂ contact reactor 106 is first convertedto very weak carbonic acid H₂CO₃ in the supernatant. The very weakcarbonic acid H₂CO₃ further dissociates in water into bicarbonate HCO₃ ⁻or carbonate CO₃ ²⁻ depending on the solution pH following the reactionsgiven in Equations (V)-(VII):

CO₂+H₂O H₂CO₃*  (V)

H₂CO₃*→H++HCO₃ ⁻  (VI)

HCO₃ ⁻→H⁺+CO₃ ²⁻  (VII)

The acidity constant for the reaction (V) is 10^(−6.3). Therefore, inthe pH range 6.5-7.5, bicarbonate HCO₃ ⁻ will be the dominating species.Sodium aluminate NaAlOH₄ in the supernatant is then converted to sodiumbicarbonate NaHCO₃ following the reaction given in Equation (VIII):

NaAl(OH)₄+H₂CO₃*→NaHCO₃+Al(OH)₃(solid)+H₂O  (VIII)

Here, H₂CO₃* denotes the sum of CO₂ and H₂CO₃. CO₂ and H₂CO₃ aredifferent species, but are not distinguishable in water. Thus, by addingCO₂ gas to the supernatant in the CO₂ contact reactor 106, the pH of thesupernatant is lowered to around 7±0.5, preferably around 7. At this pHvalue, aluminum hydroxide Al(OH)₃ or gibbsite is almost completelyinsoluble and precipitates out of the liquid as aluminum hydroxideAl(OH)₃ solid. To improve the precipitating process, some of theprecipitated gibbsite may be recycled back to the CO₂ contact reactor106 for use as nucleating seeds to produce large solids of gibbsite. Theprecipitated gibbsite has very low concentration of impurities and maybe used as coagulant in water and wastewater treatment facilities. Incase of contaminants buildup rendering the precipitated gibbsite orrecovered alum unusable for drinking water applications, the recoveredalum may be used in municipal and industrial wastewater treatmentapplications.

Upon the addition of CO₂ in the CO₂ contact reactor 106, the liquidphase from the CO₂ contact reactor 106 is primarily composed of sodiumbicarbonate (NaHCO₃) and after alum precipitation the supernatant fromthe CO₂ contact reactor 106 becomes saturated with sodium bicarbonateNaHCO₃. Furthermore, since the acidity constant for the reaction (V) isabout 10^(−6.3), in a pH ranging from 6.5 to 7.5, sodium bicarbonateNaHCO₃ will be the dominating species in the saturated supernatant.

The system shown in FIG. 1 may operate in a continuous flow mode, inwhich the alum containing sludge and the hydrated lime slurry arecontinuously fed into the sludge thickener 102 and the aluminum recoveryreactor 104, respectively, and an excess water or liquid blowdown streamfrom the CO₂ contact reactor 106 is continuously pumped out to maintainconstant liquid and solid volumes in the sludge thickener 102, thealuminum recovery reactor 104 and the CO₂ contact reactor 106,respectively.

The sodium bicarbonate NaHCO₃ produced in the CO₂ contact reactor 106may be recycled back to the aluminum recovery reactor 104 by a pump 114for aluminum solubilization to minimize the use of fresh soda ashfeedstock, thereby resulting in additional cost-saving. The pump 114 maybe a centrifuge type water pump. The excess water or liquid blowdownstream pumped out from the CO₂ contact reactor 106 may be split into twostreams. One stream is sent out to a sewage treatment plant for furtherpurification; the other one forms NaHCO₃ recycle stream going back tothe aluminum recovery reactor 104. The produced sodium bicarbonateNaHCO₃ reacts with Ca(OH)₂ in the aluminum recovery reactor 104 to formNaOH that reacts with CaAl₂(OH)₈ to form NaAl(OH)₄, thereby loweringsodium carbonate Na₂CO₃ feedstock consumption in the process. Thereaction of NaHCO₃ reacts with Ca(OH)₂ is given in the followingEquation (IX):

Ca(OH)₂+NaHCO₃→NaOH+CaCO₃+H₂O  (IX)

NaAl(OH)₄ is then produced by Equation (IV).

The liquid phase of the alkaline sludge formed in the aluminum recoveryreactor 104 contains NaAl(OH)₄ and the solid phase or slurry phase ofthe alkaline sludge contains a relatively low concentration of aluminum.The slurry phase from the aluminum recovery reactor 104 may be pumped toa sludge storage tank 110 and recycled back to the aluminum recoveryreactor 104 for maximizing aluminum recovery. A waste sludge with lessto no concentration of aluminum after recycling from the sludge storagetank 110 may be pumped to a filter press 112 for dewatering anddisposal. Water from the filter press 112 having a high pH value, forexample, a pH value ranging from 11.5 to 12, may be pumped back to thecontainer 108 for reuse as a composition for the hydrated lime slurrypreparation and the solid, dewatered sludge, from the filter press 112may be sent out for disposal.

Several water or liquid pumps (not shown) are used to wherever needed tomove sludge or liquids within the system, for example, between thesludge thickener 102 and the aluminum recovery reactor 104, between thealuminum recovery reactor 104 and the sludge storage tank 110, etc. Oneof ordinary skill in the art will recognize that the water or liquidpumps used herein including, but are not limited to, centrifuge typewater pumps.

The disclosed methods for recovery aluminum hydroxide show aluminumhydroxide may be recovered under alkaline conditions. Unlike the widelyaccepted acidulation method for recovery aluminum from the wastewatertreatment sludge, the use of hydrated lime or calcium hydroxide Ca(OH)₂and soda ash or sodium carbonate Na₂CO₃ in the aluminum recovery reactor104 and the use of CO₂ gas in the CO₂ contact reactor 106, instead ofmineral acids such as sulfuric or hydrochloric acid, have flowingadvantages.

First, as described in EP2452924 to Rossi et al., sodium hydroxide isefficient to form an alkaline sludge (or eluate), but sodium hydroxidehas a relatively high cost. Lime is less expensive, however, using limeinstead of sodium hydroxide for alum recovery has not been successfuldue to the limited solubility of calcium aluminate CaAl₂(OH)₈. In thedisclosed methods, adding soda ash or Na₂CO₃ allows CaAl₂(OH)₈ beingconverted to soluble NaAl(OH)₄. Thus, the disclosed methods overcome theproblem of CaAl₂(OH)₈, enabling the use of less expensive soda ash tothereby create cost-saving.

Second, CO₂ allows recovering NaHCO₃ for reuse in the aluminum recoveryreactor 104 without buildup of sulfate or chloride.

In addition, some of the natural organic matter (NOM) and portion ofsome mineral elements, such as iron, copper, chromium, and manganese,which were precipitated in the coagulation process, may also solubilizewhen the pH of the sludge is raised with hydrated lime for alumrecovery. Even though the concentration of these impurities may be lowcompared to recovered aluminum concentration, their concentration willincrease with each recycle of sludge from the sludge storage tank 110and sodium bicarbonate solution from the CO₂ contact reactor 106. Thesedissolved impurities may carry over to the CO₂ contact reactor 106 andprecipitate along with aluminum hydroxide. However, hydrated lime usedin the disclosed methods as the main media for raising the pH for alumrecovery in the presence of carbonate alkalinity yields CaCO₃ which isknown to aid the precipitation of large particles by inducing sweepcoagulation. To minimize the carryover of dissolved impurities to theCO₂ contact reactor 106, Mg(OH)₂ may be added to the aluminum recoveryreactor 104. Mg (OH)₂ has been shown to have good coagulation effect athigh pH values. Unless additional steps are taken to precipitate theseimpurities, the applicability of the recovered alum for drinking waterapplications might be limited, but the recovered alum may be used inwastewater clarification or for phosphate removal from wastewatertreatment plant effluent.

Another advantage of the disclosed methods for high pH aluminum recoveryis sludge disinfection. At pH values above 11.2 in the aluminum recoveryreactor 104 a complete inactivation of most if not all relevant wastewater pathogens are achieved (see Grabow et al. “Role of Lime Treatmentin the Removal of Bacteria, Enteric Viruses, and Coliphages in aWastewater Reclamation Plant”, Applied and Environmental Microbiology35.4, 663-9, 1978; and Polprasert et al. “The Inactivation of FaecalColiforms and Ascaris Ova in Faeces by Lime”, Water research 15.1, 31-6,1981). Therefore, an advantage of the methods described herein includesin some embodiments the elimination of otherwise required sludge orliquid disinfection processes, thus creating additional cost-saving.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment may be included in at least one embodiment of theinvention. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments necessarilymutually exclusive of other embodiments. The same applies to the term“implementation.”

As used in this application, the word “exemplary” is used herein to meanserving as an example, instance, or illustration. Any aspect or designdescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other aspects or designs. Rather, use ofthe word exemplary is intended to present concepts in a concretefashion.

Additionally, the term “or” is intended to mean an inclusive “or” ratherthan an exclusive “or”. That is, unless specified otherwise, or clearfrom context, “X employs A or B” is intended to mean any of the naturalinclusive permutations. That is, if X employs A; X employs B; or Xemploys both A and B, then “X employs A or B” is satisfied under any ofthe foregoing instances. In addition, the articles “a” and “an” as usedin this application and the appended claims should generally beconstrued to mean “one or more” unless specified otherwise or clear fromcontext to be directed to a singular form.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“About” or “around” or “approximately” in the text or in a claim means±10% of the value stated.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing i.e.anything else may be additionally included and remain within the scopeof “comprising.” “Comprising” is defined herein as necessarilyencompassing the more limited transitional terms “consisting essentiallyof” and “consisting of”; “comprising” may therefore be replaced by“consisting essentially of” or “consisting of” and remain within theexpressly defined scope of “comprising”.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

It will be understood that many additional changes in the details,materials, steps, and arrangement of parts, which have been hereindescribed and illustrated in order to explain the nature of theinvention, may be made by those skilled in the art within the principleand scope of the invention as expressed in the appended claims. Thus,the present invention is not intended to be limited to the specificembodiments in the examples given above and/or the attached drawings.

We claim:
 1. A method for recovery of aluminum hydroxide Al(OH)₃coagulant from an aluminum enriched water/wastewater treatment sludge,the method comprising the steps of: a) adding a hydrated lime slurry tothe aluminum enriched water/wastewater treatment sludge to form analkaline sludge; b) adding sodium carbonate Na₂CO₃ to the alkalinesludge to form a Na₂CO₃ treated sludge; c) forming a first supernatantfrom the Na₂CO₃ treated sludge of step b) containing NaAl(OH)₄; d)introducing CO₂ to the first supernatant to form a precipitate ofAl(OH)₃ and a second supernatant containing NaHCO₃; and e) recycling atleast a portion of the NaHCO₃ from the second supernatant back to thealkaline sludge of step a).
 2. The method of claim 1, wherein the pH ofthe aluminum enriched water/wastewater treatment sludge is approximately7.
 3. The method of claim 1, wherein the pH of the alkaline sludgeranges from approximately 11.5 to
 12. 4. The method of claim 1, whereinthe pH of the second supernatant ranges from approximately 6.5 to 7.5.5. The method of claim 1, wherein the pH of the second supernatant isapproximately
 7. 6. The method of claim 1, further comprising the stepof: a1) retaining the aluminum enriched water/wastewater treatmentsludge for a contact time with the hydrated lime slurry sufficient inlength to inactivate at least a portion of bacteria and viruses presentin the aluminum enriched water/wastewater treatment sludge during thestep a) of forming the alkaline sludge.
 7. The method of claim 6,wherein the contact time is about 2 to 5 hours.
 8. The method of claim1, wherein the aluminum enriched water/wastewater treatment sludgecontains up to 5% solids.
 9. The method of claim 8, wherein the solidsinclude aluminum hydroxide.
 10. The method of claim 1, furthercomprising the step of thickening the aluminum enriched water/wastewatertreatment sludge to increase a solid content of the sludge.
 11. Themethod of claim 10, wherein the thickened aluminum enrichedwater/wastewater treatment sludge contains up to 5% solid content. 12.The method of claim 11, wherein the solid content in the thickenedaluminum enriched water/wastewater treatment sludge includes aluminumhydroxide.
 13. The method of claim 12, wherein the pH of the thickenedaluminum enriched water/wastewater treatment sludge is approximately 7.14. The method of claim 1, further comprising the steps of: recycling aslurry phase of the alkaline sludge; dewatering a waste sludge from theslurry phase to form a dilution water with a pH value of 11.5 orgreater; and mixing the dilution water with a hydrated lime to form thehydrated lime slurry.
 15. The method of claim 1, further comprising thestep of adding Mg(OH)₂ to the alkaline sludge in an amount sufficient tocoagulate natural organic matter present in the alkaline sludge.
 16. Themethod of claim 1, wherein the hydrated lime slurry contains 5-10%Ca(OH)₂.
 17. The method of claim 1, wherein the alkaline sludge containscalcium aluminate CaAl₂(OH)₈ according to the reaction:2Al(OH)₃+Ca(OH)₂→CaAl₂(OH)₈.
 18. The method of claim 17, whereinNaAl(OH)₄ in the first supernatant is formed by the following reactions:CaAl₂(OH)₈+Na₂CO₃→2NaAl(OH)₄+CaCO₃Ca(OH)₂+Na₂CO₃→2NaOH+CaCO₃2NaOH+CaAl₂(OH)₈→2NaAl(OH)₄+Ca(OH)₂.
 19. The method of claim 18, whereinNaHCO₃ in the second supernatant is formed by the following reactions:CO₂+H₂O→H₂CO₃*H₂CO₃*→H⁺+HCO₃ ⁻HCO₃ ⁻→H⁺+CO₃ ²⁻NaAl(OH)₄+H₂CO₃*→NaHCO₃+Al(OH)₃(s)+H₂O, wherein H₂CO₃* denotes the sumof CO₂ and H₂CO₃ present in the second supernatant.
 20. The method ofclaim 17, wherein NaHCO₃ recycled back to the alkaline sludge formsNaAl(OH)₄ by the following reactions:Ca(OH)₂+NaHCO₃→NaOH+CaCO₃+H₂O2NaOH+CaAl₂(OH)₈→2NaAl(OH)₄+Ca(OH)₂.